Galvanized steel plates have superior resistance to rust and corrosion. For that reason, recently, galvanized steel plates have been used for automotive parts, architectural steel frames, and other members. Such galvanized steel plates are welded in accordance with their applications.
A conventional laser welding method described in PTL 1 will be described with reference to FIG. 6A to FIG. 7B. FIG. 6A and FIG. 7A are perspective views to explain a conventional laser welding method. FIG. 6B is a cross section view related to FIG. 6A, which explains the conventional laser welding method; FIG. 7B is a cross section view related to FIG. 7A, which explains the conventional laser welding method.
As illustrated in FIG. 6A and FIG. 6B, anticorrosive steel plate 101 and anticorrosive steel plate 102 are stacked on each other. Anticorrosive steel plate 101 is steel plate 111 in which galvanized layer 120a is formed on a front surface and galvanized layer 120b is formed on a rear surface. Anticorrosive steel plate 102 is steel plate 112 in which galvanized layer 120a is formed on a front surface and galvanized layer 120b is formed on a rear surface. Anticorrosive steel plate 101 and anticorrosive steel plate 102 are stacked on each other in such a manner that their galvanized layers 120a are in contact with each other.
As illustrated in FIG. 6A and FIG. 6B, stacked anticorrosive steel plates 101, 102 are irradiated with laser beam 100 as a first laser irradiation. For this first laser irradiation, a heat input quantity of laser beam 100 is set such that laser beam 100 reaches galvanized layers 120a of anticorrosive steel plate 101 and anticorrosive steel plate 102 but does not penetrate anticorrosive steel plate 102.
As a result of the first laser irradiation, zinc contained in galvanized layer 120a is melted and released to an outside. Due to the evaporation of zinc, porosities 116 may be created in bead 115 formed by the first laser irradiation.
Subsequent to the first laser irradiation, as illustrated in FIG. 7A and FIG. 7B, anticorrosive steel plates 101, 102 are irradiated with laser beam 100 as a second laser irradiation. For the second laser irradiation, the heat input quantity of laser beam 100 is set such that laser beam 100 reliably reaches anticorrosive steel plate 102, finishing welding. Therefore, the energy of laser beam 100 for the second laser irradiation is set to be higher than the energy of laser beam 100 for the first laser irradiation. Further, the energy of laser beam 100 for the second laser irradiation is required to be set to be substantially as high as the energy of a laser beam used for typical laser welding. Moreover, laser beam 100 for the second laser irradiation may pass through the same path as laser beam 100 for the first laser irradiation.
When laser irradiation is performed twice as described above, steel plate 111 melted by the second laser irradiation fills in porosities 116 that have been created by the first laser irradiation. Thus, this two-time laser irradiation can appropriately form bead 125 having no porosities 116, because the first laser irradiation removes zinc from galvanized layers 120a and then the second laser irradiation fills porosities 116 without causing zinc to be evaporated.